AFFECTING THE PILING-TJP OF VOLCANIC CONES. 741 



vulcanists will have to yield up their fruit before our knowledge 

 of even the external and visible phenomena of volcanic action can be 

 rescued from the region of guesswork and of assumption, and 

 explained without lacunae, by the acknowledged laws of matter and 

 motion as part of the physique du globe. This much, however, we 

 are so far probably justified in affirming, that the channels leading 

 to existing volcanic foci, very commonly, if not generally, reach the 

 surface through comparatively recent rock formations, and through 

 stratified rocks the beds of which are seldom of great thickness — 

 rocks which, considered lithologically, are inferior in hardness, 

 rigidity, and cohesion, and which are occasionally found to be inter- 

 calated with formations presenting still less mechanical resistance to 

 forces externally applied. To these general facts there are, however, 

 remarkable local exceptions. Thus the volcanic cones or puys of 

 Auvergne appear for the most part to be based upon an enormous 

 tabular granite formation of unknown depth, through which the 

 volcanic ducts or channels had reached the surface without serious 

 dislocation or breaking-up to any considerable extent of the rigid 

 material through which they have pierced, by a mechanism as to 

 the nature of which we had best confess our ignorance ; for we have 

 no evidence of a trustworthy character to indicate whether those 

 ducts which have reached the surface have followed between the 

 jaws of preexisting fissures, and where the resistance of the solid 

 granite had previously been weakened, or whether at certain local 

 spots hypogean heat, whencesoever originating, had gradually 

 reached the surface, more or less disintegrating and softening the 

 rock in its progress upwards by conductivity, until at length the 

 softening and breaking-up by heat, extending from the more or less 

 deep foci of fusion to the surface, had advanced so far that the 

 admission of water, whether meteoric or already deposited and 

 drained from river-channels or lakes and reaching the interior 

 through fissures, commenced that explosive action by which the 

 volcanic duct was opened to the air and the heaping-up of 

 ejecta into a cone commenced. Many circumstances connected with, 

 the puys of Auvergne, more especially the general uniformity and 

 fine comminution of the material of which they consist, as well as 

 their extinction after a brief period of activity, induced me to con- 

 jecture that they were thus originated, and that the material of the 

 respective cones consisted of tufaceous or other disintegrated 

 material preexisting at no great depth beneath the huge table of 

 granite through which it has been eviscerated. Unless we except 

 the case of Monte Nuovo in the Phlegrasan Fields, no volcano has 

 ever been seen by any competent observer in the act of bursting 

 forth. Even in this solitary instance, although the facts recorded 

 by the only two personal witnesses who have left a record of their 

 observations of the outburst make quite clear the leading phe- 

 nomena of the new birth of the mountain, some of the state- 

 ments as to minor phenomena throw some doubt upon their com- 

 petence as observers. 



However, no record exists as to the state of the ground prior to 



Q.J.G.S. No. 132. 3 c 



